Energy has become a primary focus throughout the world. There has been great interest in developing more efficient energy-storage devices. A supercapacitor is a type of energy-storage device with a broad spectrum of potential uses. In particular, supercapacitors have great potential for applications that require a combination of high power, short charging time, high cycling stability, and long shelf life.
Supercapacitors, also known as ultracapacitors or electrochemical capacitors, generally can achieve capacitances several orders of magnitude larger than conventional capacitors. Supercapacitors are thus able to attain greater energy densities while still maintaining the characteristic high power density of conventional capacitors.
The performance of a supercapacitor can be evaluated in terms of its energy density, the amount of energy that can be stored per unit volume; and in terms of its power density, the rate at which an amount of energy can be transferred in or out of that unit volume. Supercapacitors, generally speaking, have several advantages over electrochemical batteries and fuel cells, including higher power density, shorter charging times, and longer cycle life. Despite greater capacitances than conventional capacitors, supercapacitors have yet to match the energy densities of many batteries and fuel cells.
Conventional capacitors have relatively high power densities, but relatively low energy densities when compared to electrochemical batteries and to fuel cells. That is, a battery can store more total energy than a capacitor, but it cannot deliver it very quickly—which means its power density is low. Capacitors, on the other hand, store relatively less energy per unit mass or volume, but what electrical energy they do store can be discharged rapidly to produce a lot of power—which means their power density is usually high.
Supercapacitors are generally known in the art. Yet, many of the emerging applications for supercapacitors require them to be very small, such as about 100 μm or less in any dimension. Such very small supercapacitors may be referred to as micro-supercapacitors. Typical micro-supercapacitors employ cell separators which take up valuable cavity space in a sub-100 μm size device, and thus reduce the energy capacity of the supercapacitor.
There remains a need for micro-supercapacitors with high energy densities and high power densities, resulting from fast ionic and electronic capacitance. Preferably, the micro-supercapacitor contains no separator, to maximize working volume and energy density. What is desired is a micro-supercapacitor that can be fabricated and hermetically sealed for practical use in a variety of applications.